1. Field of the Invention
The present invention relates to a new class of polymerizable Type II photoinitiators, especially suitable for food compliant radiation curable formulations.
2. Description of the Related Art
A free radical photoinitiator is a chemical compound that initiates a polymerization of monomers when exposed to actinic radiation by the formation of a free radical. Photoinitiators are frequently used in UV-curable compositions, such as UV curable inkjet inks.
Two types of free radical photoinitiators can be distinguished. A Norrish Type I initiator is an initiator which cleaves after excitation, yielding the initiating radical immediately. A Norrish type II-initiator is a photoinitiator which is activated by actinic radiation and forms free radicals by hydrogen abstraction from a second compound that becomes the actual initiating free radical. This second compound is called a polymerization synergist or co-initiator.
When radiation curable compositions are used for food packaging, toys and dental applications, the amount of extractable residues is a critical issue and needs to be minimized. Low molecular weight products are usually not completely built into the polymer network and are prone to be readily extracted or to diffuse out of the cured composition.
Especially Norrish type II initiators are a point of concern regarding extractable residues. Norrish type II photo-initiators always require a co-initiator. Aliphatic tertiary amines, aromatic amines and thiols are preferred examples of co-initiators. After transfer of a hydrogen atom to the Norrish type II initiator, the radical generated on the synergist initiates the polymerization. Theoretically the co-initiator is built into the polymer network. However, it is highly unlikely that both the hydrogen transfer and the initiation reaction yields are a hundred percent. Side reactions are likely to occur leaving unreacted synergist and side products in the composition. In food packaging printed upon with such a radiation curable composition, these low molecular weight residues remain mobile and if toxic will cause health risks upon being extracted into the food.
One approach to minimize extraction of the photoinitiator is to use a photoinitiator having one or more ethylenically unsaturated polymerizable groups so that it can be copolymerized with the other monomers of the radiation curable composition. However the copolymerization reduces the mobility of the photoinitiator and a reduction in curing speed can be observed.
JP 2004-224993 (NIPPON KAYAKU) discloses self-photopolymerization type photopolymerization initiators used in radiation curable compositions for reducing its evaporation from a cured film.
Another approach in solving the extraction problem is to design Norrish Type II initiators with a higher molecular weight. However by using these photoreactive polymers, the solution viscosity often increases to an undesirable level for a great number of applications with radiation curable compositions, e.g. inkjet inks and lacquers.
EP 1674499 A (AGFA GRAPHICS) discloses radiation curable compositions and photoreactive polymers including a dendritic polymer core with at least one initiating functional group and at least one co-initiating functional group. While the use of a dendritic polymer core is advantageous for maintaining a low viscosity of the radiation curable composition, an improvement in curing speed is still desirable, especially in the absence of nitrogen inertisation.
Therefore, there still remains a need for photoinitiators, combining a high reactivity, without the need for nitrogen inertisation and a low impact on the viscosity of the formulation, while still maintaining a low amount of extractable residues.